This invention relates generally to integrated services digital networks (ISDN) and, in particular, to ensuring a voice telephone service at a customer's location when a power failure occurs at the customer's location.
Recently there has been a strong trend toward development of an integrated services digital network, which will either replace or coexist with traditional telephone service.
ISDN signal in a customer loop is sent over the same pair of wires which has been used in the past for plain old telephone service (POTS). ISDN, however, utilizes a coded digital signal with higher information density in both directions of transmission and, by doing so, increases the channel capacity. In particular, a single ISDN channel provides two voice frequency (VF) channel service plus a 16 kb/s data service (2B+D).
A 64 kb/s digital Pulse Code Modulated (PCM) signal is required for each VF channel; consequently, two VF channels plus 16 kb/s data channel requires 144 kb/s digital signal. ISDN can be used to provide multiple data channels instead of any VF channel, as long as the total bit rate doesn't exceed 144 kb/s. A detailed description of the ISDN can be found in literature, e.g., Anthony M. Rutkowski's "Integrated Services Digital Networks," 1985, Artech House, Inc.
Complex coding/decoding equipment serving two telephone and data channels requires much more power than a regular single telephone, therefore, at the customer premises local power is required for the ISDN equipment. Consequently, local power failures will not only eliminate the subscriber's data service, but will also eliminate regular voice service.
A typical ISDN service consists of three channels, referred to as two B channels plus a D channel. The two B channels are intended either for VF service or data up to 64 kilobits per second each. The D channel is a 16 kb/s data channel. The D channel could also be used for a low speed data transfer, such as remote meter reading at the customer's location or similar services. Typically, for a single pair of wires at a customer location, the ISDN type service would provide two voice lines, plus data services, such as data transfer of telemetry, e.g., meter reading. Alternatively, the customer could have one voice line, one high-speed data line and one medium-speed data line. Since this system requires intelligence consuming substantial power at both ends, it also requires local power at both ends of the link to operate the equipment.
It is well known that at times of local power failures people know that their phone will still operate and that they can use the telephone to make emergency calls when necessary. In an ISDN system, this will not be the case because once local power is lost, both of the B lines, as well as the D line, are inoperative.
In order to overcome this problem in similar applications in the prior art, one solution has been to provide a battery to power the local equipment so that at least the customer would be able to use the voice line even though the data lines may be inaccessible. Another approach in the prior art is to provide redundant facilitates in a sense that another pair of wires is connected to the customer location to provide the plain voice telephone service. Both such solutions are very costly and complex.
The present invention overcomes this problem in the prior art by utilizing an ISDN system while continuing to supply voice service to the customer location in the event of a power failure. The present invention accomplishes this without the use of redundant facilitates and without the use of batteries or portable generators to run local equipment. Furthermore, the present invention provides that the voice link remains present even when the failure is within the digital terminal equipment at the customer's location, as opposed to a power failure.